Artifact Beam hardening

The patient should he positioned as comfortably as possible for any examination. For this purpose,special headrests, arm supports (particularly for supporting the arms in the above-head position), and cushionlike supports for the knees are available. The patient must be positioned on a soft surface, which should in addition be heat-insulated. Because most CT scanners are no longer water-cooled but dissipate their heat directly into the room air and because the scanner rooms must normally be fully air-conditioned for this reason, the patient should be covered with a protective sheet. None of the materials employed should contain silicate, glass or other absorbing substances, since the presence of these substances in the scan field can lead to artifacts or increased image noise as a result of beam hardening. 

Beam hardening artifacts (for polychromatic X-rays or neutrons) prevent exact quantification of the attenuation coefficients. 

The appearance of cupping and shadowing artifacts due to scatter can be very similar to those created by beam hardening (Ruehrnschopf and Kalender 1981). Contrary to beam hardening, however, a decrease in differential contrast and an increase in noise are typical consequences of scatter. 

Spectral CT could therefore have a major impact on the clinical value of CT coronary angiography. Without beam hardening artifacts, in-stent lumens become accessible and with partial volume averaging, reduced both vessel lumens adjacent to stent struts and densely calcified plaques are more accurately dehn-eated (Feuerlein et ah 2008). Overall, two new application scenarios could be feasible one is the foUow-up of patients after percutaneous coronary interventions with stent implantation the other is the evaluation of patients with known CAD or calcified vessels. Spectral detector technology may promote CT to become the primary tool for CAD imaging (Roessl and Proksa 2007 Feuerlein et al. 2008). 

We were trying to apply this technique in a variety of different settings. We realized the advantage of C-arm CT in case of aortic dissection in order to follow the course of the catheter through the false and true lumen (Fig. 41.1). Also, in case of both sided hip en-doprotheses where beam-hardening artifacts between the two metal pieces hinders conventional CT visualization of the structures in the small pelvis, C-arm CT was able to demonstrate the proximity of the internal iliac... 

Fig. 41.1a-d. Beam-hardening causes severe artifact in the region of the small pelvis in conventional multi-slice CT (a). Because C-arm CT is based on projections, these artifacts are less likely to occur (b). Three-dimensional volume-rendering... 

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